Epoxidation of alkenes is an important chemical transformation whereby an oxygen atom is added to a carbon-carbon double bond to form an epoxide. Epoxides are often utilized as intermediate compounds which can then be transformed to final products. Examples include but are certainly not limited to ethylene glycol and polyethylene glycol from ethylene oxide, propylene glycol from propylene oxide, phenylacetaldeyhyde from styrene oxide and propranolol from 2R-glycidol.
Epoxidation of alkenes can be carried out using numerous techniques. The oldest and probably most common method is to react the alkene with an organic peracid, according to the reaction set forth in equation (1). ##STR1##
Typical peracids used in the art include perbenzoic acid, peracetic acid, performic acid, perphthalic acid and substituted perbenzoic acids such as 3-chioroperbenzoic acid. The salts of such acids may also be effective oxidants as in the case of magnesium monoperoxophthalate. The adds may be used as pure compounds or as prepared in situ in the reaction mixture by for example adding hydrogen peroxide to acetic anhydride to form peracetic acid. Although processes based on the reaction as described in equation (1) are known, there are certain drawbacks that are associated with such reactions. Among these one may site (a) the propensity for formation of by-products such as glycols and glycol esters by reaction of the epoxide with water and/or acid in the reaction medium, (b) the necessity of recovering and/or recycling the add co-product and (c) the necessity for stringent reaction control because of the safety danger involved in use of organic peracids (acyl hydroperoxides).
In order to minimize the danger in using peroxides as oxidants the use of alkyl and alkylaryl hydroperoxides in place of acyl hydroperoxides has been suggested and applied. These oxidants do not normally react with alkenes and the addition of a catalyst is required as shown in the reaction illustrated in equation (2). ##STR2##
Some hydroperoxides commonly used in such reactions are tert-butylhydroperoxide, cumene hydroperoxide and ethylbenzene hydroperoxide. The catalysts used are most commonly based on compounds containing Ti(IV), V(V), Mo(VI) or W(VI) although many compounds based on other metals have been described as being effective catalysts. These reactions are safer because of the lower reactivity of alkyl and alkylaryl hydroperoxides compared to organic peracids, however, the other disadvantages associated with the use of acyl hydroperoxides remain. Thus, reactions are not necessarily more selective, since the presence of catalysts often leads to additional side reactions, for example, substitution and oxidation at the allylic carbon of the alkene instead of oxygen addition to the double bond. Similar to the problems encounter with the use of acyl hydroperoxides, the alcohol co-product must be recovered, recycled and/or otherwise utilized.
A further method to epoxidize alkenes is to use aqueous hydrogen peroxide as oxidant as shown in the reaction illustrated in equation (3). ##STR3##
Such a reaction represents a conceptual improvement compared to the use of organic hydroperoxides in that the co-product is water and therefore is environmentally benign and need not be recovered or recycled. As in the use of alkyl- and alkylaryl hydroperoxides the presence of a catalyst is necessary, which catalysts are again often compounds containing Ti(IV), V(V), Mo(VI) or W(VI), among others. In only certain cases has high selectivity been reported for alkene epoxidation. Some effective and selective catalysts include titanium silicate-1 and other titanium substituted zeolites, and polyoxometalates such as [WZnMn.sub.2 (ZnW.sub.9 O.sub.34).sub.2 ].sup.12- and {PO.sub.4 [WO(O.sub.2).sub.2 ].sub.4 }.sup.3-. In many cases, the use of hydrogen peroxide represents an ideal oxidant provided reactions are selective. An exception is in cases where the low price of the epoxide make the use of hydrogen peroxide prohibitively expensive.
An additional important method for synthesis of epoxides from alkenes is via formation of a halohydrin, preferably a chlorohydrin, using hypochlorous acid in the first step, followed by use of base eg NaOH for ring closure in the second step, as shown in the reaction illustrated in equation (4). ##STR4##
This is a very simple procedure which has, however, two problems. First, usually the presence of molecular chlorine in hypochlorous acid leads to formation of dichlorinated organics which are undesirable by-products and must be disposed of. Second, the process also forms large amounts of salts as co-product which also must be treated or recycled.
The ideal oxidant for alkene epoxidation both from an ecological and economic point of view would be molecular oxygen (dioxygen) as found in air. The addition of dioxygen to an alkene is disfavored kinetically, thus catalytic procedures need to be applied. In cases where there is no allylic carbon to the double bond, oxygen may be added to the double bond using a silver catalyst at elevated temperatures. In this way, ethylene oxide is manufactured from ethylene. For similar procedures with other alkenes, such as 1-butene, propene etc. this reaction fails to give epoxide in significant amounts. The basic problem in use of dioxygen for epoxidation of alkene lies in the radical nature of the molecular oxygen molecule. In homogenous reactions, this radical nature always leads to a preferred radical reaction via substitution of hydrogen at an allylic carbon atom. Therefore, the common mode of utilization of dioxygen in liquid phase catalyzed reactions does not yield epoxide as a major product The situation in gas phase reactions is similar wherein activation of alkenes leads to allylic type carbocations, carbanions or carbon radicals again preventing formation of epoxides as a significant product.
Conceptually, in order to use dioxygen for alkene epoxidation, activation of dioxygen should be via formation of a high valent metal oxo compound formed after scission of the oxygen-oxygen bond. These high valent metal-oxo intermediates are effective epoxidizing agents. Most commonly this is carried out in nature by use of monoxygenase type enzyme such as cytochrome P450 or methane monoxygenase. Such enzymes may be mimicked, for example, by using manganese and iron porphyrins as catalysts. The monooxygenase mechanism, however, requires two electrons from a reducing agent in order to cleave the oxygen-oxygen bond leading to formation of the high valent metal-oxo intermediate active in alkene epoxidation. From a process point of view the reducing agent becomes the limiting reagent instead of dioxygen and negates the attractivity of such a process.
The alternative is activation of dioxygen in a dioxygenase type mechanism. In such a reaction, dioxygen is cleaved using two metal centers leading to formation of two high valent metal-oxo species. This type of reaction has been only realized using a ruthenium substituted tetramesitylporphyrin (RuTMP). Turnover rates to epoxide are very low and the catalyst has limited stability.
The limited stability of porphyrin ligands has led to the suggestion that transition metal substituted polyoxometalates may be important alternative catalysts to metalloporphyrins as disclosed and discussed in Hill, U.S. Pat. No. 4,864,041. These catalysts would retain the high activity of their metalloporhyrin counterparts, however, are significantly more thermally and oxidatively stable, thus allowing their use as long living catalysts. This previous work describes the application of transition metal substituted polyoxometalates for the epoxidation of alkenes using oxygen donors such a iodosylbenzene. Other reported academic research has evolved from this report and has described alkene epoxidation using other oxygen donors such as tertfutyihydroperoxide, hydrogen peroxide and p-cyano-N,N-dimethylaniline-N-oxide. The use of transition metal substituted polyoxometalates as catalysts for alkene epoxidation with molecular oxygen: has never been described.